1. Field of the Invention
The present invention relates to a fuel cell for use in, for example, a portable power supply, a power supply for an electric car, a household co-generation system, and other such devices and systems, and a process for the production thereof.
2. Related Art of the Invention
The basic structure of the related art polymer electrolyte type fuel cell will be described in connection with FIG. 7. FIG. 7 is a longitudinal sectional view illustrating the configuration of the related art polymer electrolyte type fuel cell stack.
A solid polymer electrolyte type fuel cell allows a fuel gas containing hydrogen and an oxidizer gas containing oxygen, such as air, to undergo an electrochemical reaction to generate electricity and heat at the same time. The solid polymer electrolyte type fuel cell of FIG. 7 comprises a polymer electrolyte membrane (PEM) 71, which allows the selective transportation of a hydrogen ion upon the application of an electric field, and a pair of gas diffusion electrodes 72 formed on the respective sides of the polymer electrolyte membrane 71.
Gas diffusion electrode 72 mainly comprises a carbon powder, having a platinum group metal catalyst supported thereon, and comprises (1) a catalyst layer formed in contact with polymer electrolyte membrane 71 and (2) a gas diffusion layer, having both air permeability and electronic conductivity, that is formed on the side of electrode 72 opposing a separator 73. Also, a gasket 74, or gas sealing material, is disposed around gas diffusion electrode 72 as shown to prevent the two supplied gases from leaking or being mixed with each other. Gasket 74 may be pre-assembled integrally with gas diffusion electrode 72 and polymer electrolyte membrane 71 (in a one-piece structure or coupled therewith).
The configuration comprising polymer electrolyte membrane 71 and the electrode 72 is referred to as a membrane-electrode assembly (MEA). In an ordinary polymer electrolyte type fuel cell, the MEA is mechanically fixed and an electrically-conductive separator 73 is provided for electrically connecting adjacent MEAs to each other in series. The lamination of a number of single cells, essentially comprising the MEA and electrically-conductive separator 73, produces a fuel cell stack.
Separator 73 is made of an electrically-conductive and airtight material, having some corrosion resistance, such as a carbon plate and a metal plate. In each of the single cells of the stack, a gas channel for supplying the reactive gas onto the surface of gas diffusion electrode 72 and removing the produced gas or extra gas is formed on the portion of separator 73 in contact with the MEA. Alternatively, the gas channel may be provided on a portion of the stack other than separator 73 such as the surface of gas diffusion electrode 72. However, it is usual that a groove is provided on the surface of separator 73 to form a gas channel.
To supply the reactive gas through the gas channel, a means is required for supplying and distributing the reactive gas into the respective single cells, collecting the residual gas and the gas produced in gas diffusion electrode 72, and discharging these gases to the exterior of the cell. As such means, a hole is formed through the respective single cells to supply the fuel gas and oxidizer gas into the respective single cells and discharge these gases. The hole is referred to as a “manifold.”
Manifolds are divided into two types. An internal manifold is a series of communicating through-holes formed by laminating separators 73, each having a through-hole formed therein in the direction of the stack. An external manifold is formed on the side of a laminate of separators 73, as a structure distinct from separator 73.
A fuel cell generates heat during operation and thus is required to be cooled with cooling water, or the like, to keep itself under good temperature conditions. Generally, a cooling portion is interposed between separators 73 to enable cooling water to flow to one to three cells. In most cases, the cooling portion includes a cooling water channel provided on the back of separator 73. The supply of cooling water into the cooling portions and discharge of cooling water therefrom are conducted through the manifold, which is formed through the respective cells. The supply and discharge of the reactive gases are usually conducted through this manifold also.
An ordinary cell stack is obtained by laminating MEAs, separators 73, and cooling portions on each other to form a laminate of from 10 to 200 fuel cells. The stack is clamped between end plates, with a collector and an insulating plate interposed therebetween, and then the stack is fixed with a clamping bolt from both ends thereof.
Hydrogen is used as the fuel for the polymer electrolyte type fuel cell. Hydrogen may be supplied from a hydrogen bottle or may be obtained by converting a hydrocarbon fuel to hydrogen through a modifier. Air may be used as the oxidizer gas.
Since the polymer electrolyte membrane can be provided with a high hydrogen ionic conductivity only when it is hydrous, either the fuel gas or air supplied to the fuel cell is often provided with water vapor. For the supply of such a reactive gas into the fuel cell, a blower or compressor is used.
The electric power produced by the fuel cell is direct current (DC) power, which is better used at a higher voltage to give a higher utility. Accordingly, DC power is converted to alternating current (AC) power, having a higher voltage, by a power converter or inverter.
The electrochemical reaction of hydrogen with oxygen and the resulting generation of electric current are accompanied by the generation of heat. In order to keep the cell temperature constant, the heat thus generated is released to the exterior of the cell or the cell is cooled with a heat medium. The heat which has been withdrawn to the exterior of the fuel cell may be utilized as a hot water supply or used for heating in a household co-generation system.
A fuel cell system comprises a fuel cell such as described above, a modifier, a power management portion, such as a power converter and inverter, a heat utilization element, and a control system for functionally operating these portions.
The particular gas diffusion electrode 72 into which the fuel gas is supplied is referred to as an “anode,” and the electrode into which the oxidizer gas, such as air, is supplied is referred to as a “cathode.” During the generation of electricity, the anode acts as a negative electrode while the cathode acts as a positive electrode.
On the anode, the supplied hydrogen is oxidized in the vicinity of the catalyst to produce a hydrogen ion, which is then released into the electrolyte. On the cathode, the supplied hydrogen ion from the anode and the oxygen in the oxidizer gas react to produce water.
Accordingly, these gas diffusion electrodes 72 must be highly air-permeable throughout their entireties, so that they can be thoroughly supplied with the reactive gas. The reactive gas is thoroughly supplied onto the surface of the catalyst, which is a reaction site. Additionally, gas diffusion electrode 72 must be highly air-permeable so the water vapor that is produced by the electrochemical reaction and the unreacted carbonate gas, nitrogen, etc., can be readily discharged from the reaction site. Similarly, it is important that these gas diffusion electrodes 72 are arranged such that the hydrogen ion and an electron can be easily supplied into and discharged from the reaction site.
The supplied gas is moistened at a dew point close to the cell temperature, to enhance the hydrogen ionic conductivity of the electrolyte. Therefore, when the gas is consumed at any of the electrodes, the supersaturated water vapor undergoes dew condensation on the interior of the electrodes. The amount of water is greater on the cathode because it also contains water content produced by the electrochemical reaction. The water condensate thus formed is then re-evaporated in the supplied gas or discharged as a water droplet. The water droplet and the discharged gas are discharged into the gas discharge manifold, via the gas supply passage.
As a method of producing the MEA for a polymer electrolyte type fuel cell, there has heretofore been normally employed a method which comprises forming a polymer electrolyte membrane 71 according to an extrusion method, subjecting electrolyte membrane 71 to heat treatment, forming a catalyst layer on both sides of polymer electrolyte membrane 71 according to a printing method, transferring method or the like, and then forming a gas diffusion layer made of carbon paper, carbon cloth or the like on the outer side of the catalyst layer.
In recent years, another production method has been practiced to improve the cell performance and reduce the production cost. This method comprises casting a polymer electrolyte membrane 71 into a sheet with a polymer electrolyte solution, continuously forming an anode-side catalyst layer and a cathode-side catalyst layer on the front and rear sides of the sheet, and then subjecting the combination to heat treatment. To prevent a break of polymer electrolyte membrane 71, the membrane 71 may be provided with pores 21 or the membrane may be provided with a fiber material 22, or similar material, as shown in FIGS. 9 and 10 (see JP-A-8-162132, JP-A-8-213027, JP-A-8-329962, and JP-A-2001-3451100). FIG. 9 is a longitudinal sectional view illustrating a reinforcing arrangement for the MEA, in a related art polymer electrolyte type fuel cell. FIG. 10 is a longitudinal sectional view illustrating another reinforcing structure for the MEA, in a related art polymer electrolyte type fuel cell. The disclosures of JP-A-8-162132, JP-A-8-213027, JP-A-8-329962, and JP-A-2001-3451100 are incorporated herein by reference in their entireties.
In general, a perfluorocarbonsulfonic acid to be used as a polymer electrolyte is formed by a main chain moiety, for securing thermal and electrochemical stability and mechanical strength, and a pendant moiety which takes part in ionic conduction. When the perfluorocarbonsulfonic acid acts as an electrolyte, the pendant moieties gather together to cause hydration of water molecules that form an ionic conduction channel. To keep the ionic conductivity of the polymer electrolyte high, it is necessary that the supplied gas be moistened to keep the polymer electrolyte highly hydrous.
Generally, a polymer electrolyte has properties of a viscoelastic material. In other words, when a predetermined tension (or compressive force} is kept applied to the electrolyte membrane, the initial elastic deformation is followed by plastic deformation, i.e., so-called creep. On the contrary, when a tension (or compressive force) causing a predetermined deformation is kept applied to the electrolyte membrane, the electrolyte membrane undergoes relaxation and tension reduction (or compressive force) with time, i.e., so-called stress relaxation.
A polymer electrolyte type fuel cell comprises a stack of basic configurations, each comprising a polymer electrolyte membrane 71, gas diffusion electrodes 72 with the polymer electrolyte membrane 71 interposed therebetween, and a separator 73. These constituent parts are clamped at a predetermined pressure on both ends of the stack as shown in FIG. 7. Accordingly, a clamp providing a predetermined compressive pressure 75 is always applied to these constituents.
When a compressive pressure acts on polymer electrolyte membrane 71 from separator 73 over an extended period of time, via the catalyst layer and the gas diffusion layer, polymer electrolyte membrane 71 undergoes plastic deformation. Under such condition, because the catalyst layer and the gas diffusion layer each are essentially a porous material and have a complicated surface, part of the polymer electrolyte membrane 71, which has undergone plastic deformation, penetrates the interior of the catalyst layer or the gas diffusion layer. The penetration part is that having a relatively low density or small mechanical strength, as shown in FIG. 8. FIG. 8 is a longitudinal sectional view illustrating the cell in the related art polymer electrolyte type fuel cell stack after a prolonged operation.
Furthermore, when creep proceeds, the reactive gases on the anode and cathode sides eventually mix with each other to cause cross-leak or the anode and the cathode make an electrical contact with each other to cause minute short-circuiting as indicated by the sign X in FIG. 8. The cross-leak or minute short-circuiting not only causes cell performance deterioration, by itself, but also gives a new cause of performance deterioration due to local heat generation or drying or shortage of the reactive gases.
The clamping pressure 75 applied to the stack from both ends is supported by gasket 74 or sealing material arranged around the MEA. The contact pressure applied by separator 73 to the electrolyte membrane 71 via gas diffusion electrode 72 reaches a predetermined value. Then, polymer electrolyte membrane 71, which is a viscoelastic material, undergoes stress relaxation. Thus, the contact pressure decreases with time.
When the contact pressure across the catalyst layer and the gas diffusion layer and across the gas diffusion layer and the separator decrease, as indicated by the sign Y in FIG. 8, the contact resistance of electronic conduction increases. This increased contact resistance increases the electricity generation loss at these sites. As a result, the cell performance deteriorates.